Spread spectrum communication in its basic form is a method of taking a data signal that is used to modulate a sinusoidal carrier and then spreading its bandwidth to a much larger value, e.g. in a global positioning system (GPS) application, the spreading is achieved by multiplying a single-frequency carrier by a high-rate binary (−1,1) pseudo-random noise (PRN) code sequence known to GPS users. Thus, the signal that is transmitted includes a data component, a PRN component, and a (sinusoidal) carrier component.
At the receiver of such a signal, a synchronized replica of the transmitted PRN code is required to de-spread the data sequence. Initial synchronization, called acquisition, is followed by fine synchronization, which is called tracking.
The present invention relates primarily to acquisition, not tracking. Acquisition is the process by which the replica PRN code is synchronized (to within a small timing offset) with the code conveyed by the received signal either for the first time or after losing a previously acquired signal, and also by which the carrier frequency of the received signal is determined. Thus, to acquire a signal, an acquisition system must accurately determine any frequency-shifting of the received signal from the transmitted frequency in order to accurately wipe off (remove) the carrier signal. Frequency-shifting can be caused by relative motion of the transmitter and receiver (Doppler-shifting) as well as by clock inaccuracies (so that a transmitter and receiver sometimes do not agree on what is in fact the same frequency). The carrier frequency-shifting results in a modulation of a code component after carrier wipe-off in the receiver. Thus, in acquiring a signal, it is also necessary that the replica code sequence be not only time-aligned with the received code sequence, but also modulated to compensate for the frequency-shifting so as to fully eliminate the PRN sequence and leave behind only the data conveyed by the received signal. The acquisition process is therefore a two-dimensional search, a search both in code phase and in frequency.
For some GPS signals (the civil GPS or C/A (course acquisition) code signals), the search interval in the frequency domain can be as large as +/−6 kHz. In addition, the phase of the received code relative to the replica can be any possible value of code phase, due to uncertainties in position of the satellite and time of transmission of the received signal. A PRN code period is typically 1023 chips, the term chips being used to designate bits of code conveyed by the transmitted signal, as opposed to bits of data. Thus, the acquisition module of a receiver must search a 12 kHz-wide interval with 1023×ks different code phases, where ks denotes the number of samples per chip.
Ordinary GPS receivers, i.e. those designed only for operation with unobstructed satellites, search for the frequency shift with a granularity of around 1 kHz. Thus, such receivers must search 12×1023×ks different code/frequency combinations.
A GPS receiver designed for indoor operation must have an operating mode with equivalent noise bandwidth on the order of 10 Hz in the acquisition stage. Even with an equivalent noise bandwidth as small as 10 Hz though, for reliable tracking some post-detection filtering must still be performed as well as some further refining of the value determined for the carrier frequency in the acquisition stage. The granularity of 10 Hz requires that the receiver search 1200×ks×1023 different code/frequency combinations and makes the sequential search so time-consuming as to be unrealistic, motivating the use of parallel and fast search methods.
Because of the granularity used for the carrier frequency in the acquisition stage, the tracking stage often includes a preliminary fine frequency process for refining the carrier frequency determination, before initiating tracking. An alternative is to provide a more precise signal acquisition, but to do so is problematic, because signal acquisition is based on performing a correlation of the received signal with a replica, a correlation that includes an estimate of the offset of the carrier frequency from a nominal carrier frequency (due to the Doppler shifting and clock inaccuracies mentioned above), and since the received signal is modulated not only by the PRN code (for example at 1.023 MHz for a coarse acquisition signal) but also by the navigation message at a bit rate of 50 Hz, the correlation becomes corrupted if a signal fragment is used that is longer than 20 ms (the duration of a navigation data bit). Thus there is an apparent limit of 50 Hz for the precision with which the carrier frequency can be acquired. It is possible to overcome the apparent limit by performing multiple correlations, but then signal acquisition takes longer.
What is needed is a method of fast acquisition by a GPS receiver (or, more generally, any ranging receiver, i.e. not necessarily a ranging receiver used with the Global Positioning System but including for example ranging receivers used with GLONASS, the Russian version of a global positioning system) that includes a more precise estimate of the carrier frequency than is typically provided (i.e. to within a few Hz, as opposed to 50 Hz), and so abbreviate or eliminate the need for any preliminary fine frequency process in the tracking loop of the receiver.